Catheter-mounted ultrasonic transducers have in the past taken several forms, including (1) single-element transducer crystals that are pointed radially outward and rotated about the axis of the catheter, (2) radial phased array transducers and, (3) linear array transducers. For example, Bom U.S. Pat. No. 3,958,502 discloses a catheter with a radial phased ultrasonic array arranged circumferentially around the axis of the catheter. Proudian U.S. Pat. No. 4,917,097 describes a similar catheter with a radial phased ultrasonic array (and alludes to other geometries). Other catheters with radial phased ultrasonic arrays are described in Griffith et al. U.S. Pat. No. 4,841,977; O'Donnell et al. U.S. Pat. No. 5,186,177; and Barlow et al. U.S. Pat. No. 5,456,259. Seward et al. (Seward, J. B., D. L. Packer, R. C. Chan, M. G. Curley, A. J. Tajik (1996)), "Ultrasound Cardioscopy: Embarking on a New Journey," Mayo Clinic Proceedings, 71(7)) describe a catheter having a linear phased ultrasonic array for insertion into the heart. Eberle et al. U.S. Pat. No. 5,368,037 describes a catheter that has an ultrasound transducer array according to several embodiments. In one embodiment the array is configured as a cylinder about a cylindrical core (see FIG. 1 of the '037 patent), in another embodiment the transducer array is a side fire, side-looking linear array (see FIGS. 7A and 7B of the '037 patent) and in another embodiment the transducer array is an end fire, forward-looking array (see FIGS. 8A-C of the '037 patent).
Thus, with respect to known catheters, which incorporate ultrasonic transducer technology, only one type of transducer array is incorporated in the catheter, whether it be a radial phased array, a linear array or single crystal design.
While radial phased transducers provide good near field resolution, i.e. about 1-5 cm, a disadvantage of catheters that utilize a radial phased array is that the radial phased image format generated by the array does not have the diagnostic image resolution or penetration that a linear phased array can produce. In addition, radial phased imaging devices cannot image across the heart to opposite chambers or image larger cardiac structures or image far beyond the vessel it is placed in.
Conversely, while catheters having linear phased arrays have good resolution in the near and far fields, i.e., up to about 12 cm, and are able to image across the heart to opposite chambers, or across vessels to other organs, the location of an image plane is sometimes difficult to interpret.
Thus, it is desirable to provide a catheter incorporating ultrasonic transducer technology that allows good near and far field resolution while accurately depicting the location in the heart or vessel.
In addition, catheters that utilize a single type of ultrasonic transducer array only provide two dimensional information of the region examined by the catheter. Attempts have been made to construct three dimensional images using a catheter with a linear ultrasonic array by collecting multiple two dimensional image data frames using the array mounted on the catheter along with relative positional information among the image data frames so that these image frames may be subsequently assembled into a three dimensional volume to form the desired three dimensional reconstruction. The relative positional information is acquired by externally rotating the catheter while trying to maintain angular control. Such manual techniques are slow and cumbersome and therefore have many drawbacks.
One approach is described in the article by Gussenhoven et al., entitled "Displacement Sensing Device Enabling Accurate Documentation of Catheter Tip Position," Intravascular Ultrasound, pg. 157-166 (1993), involves incrementally inserting a catheter having a radial scanning array into a region of interest to acquire multiple spaced two dimensional radial scans while monitoring the incremental increase in depth of penetration by passing the catheter between rollers which are attached to rotary encoders. The inclusion of mechanical sensing devices, however, compromises reliability of the measurements. In addition, the rollers may slip against the surface of the catheter thereby introducing error in the measurements.
Thus it is also desirable to provide a catheter that incorporates ultrasonic transducer technology that allows three dimensional images to be constructed of the region examined by the catheter in a precise but facile manner.